1. Field of the Invention
The present invention relates an Al alloy film, an electronic device, and an active matrix substrate for use in electrooptic display devices. In particular, the present invention relates to an Al alloy film that is used for wiring, an electrode film, and the like (which are collectively called “an electrode film” hereinafter) of an electrooptic display device such as a liquid crystal display device and an organic EL (electroluminescence) display device, and an electronic device such as a semiconductor device and an active matrix substrate for use in electrooptic display devices using such an Al alloy film.
2. Description of the Related Art
An electrooptic display device using a TFT active matrix substrate in which thin film transistors (which are called TFTs hereinafter) are used as the switching elements has been known as one example of semiconductor devices. Electrooptic display devices have increasingly found application as one of alternative flat panel display devices to the CRTs (Cathode Ray Tubes) in products in which the advantageous features of the electrooptic display devices such as low power consumption and low profile are fully exploited.
The electrode film used in such TFT active matrix substrates are required to be electrically connected to a TFT semiconductor layer that is made of a Si film or contains a Si film as the main component, or to a transparent oxide conducting layer (e.g., ITO) used for a pixel electrode film, a terminal pad, and the like. Therefore, the so-called high melting point metal material such as titan (Ti), chromium (Cr), molybdenum (Mo), tantalum (Ta), tungsten (W), and alloys having these metals as the main ingredients has been used as typical electrode film material in the related art. That is because these high melting point metals have almost no interfacial diffusion reaction in the bonding interface with the Si semiconductor film, and have excellent electrical contact characteristics in the bonding interface with the oxide conducting film such as ITO.
However, as flat panel display TVs have been becoming larger and the resolution of compact displays such as the displays of mobile phones has been becoming higher, the need to reduce the electrical resistance of wiring material has arisen. Therefore, the specific resistances of high melting point metal in the related art (typically 12-60 μΩ·cm) have no longer been considered to be sufficient. Furthermore, in the case of a reflective display in which images are displayed by using reflected light, the electrode film is required to have high light reflectivity to achieve a brighter display characteristic. However, the reflectivity of high melting point metal in the related art has been typically around 60% and not considered to be sufficiently high. In view of such circumstances, aluminum (Al), which has low specific resistance and high light reflectivity and is easy to make wiring pattern, or Al alloy films composed of alloys containing Al as the main ingredients have become a focus of attention as the electrode film material for display devices.
However, some disadvantages are known to arise when an Al alloy film in the related art is applied to the electrode film of a TFF active matrix substrate like one described above, and thereby the application has been very difficult. That is, Al alloy films in the related art are, firstly, known to have poor heat resistance. For example, when they are heated above 100° C. in the manufacturing process of devices, protrusions called “hillocks” and “whiskers” appear on the film surface and the edge surface. These protrusions may cause defective coverage of the protective insulating film formed in the upper layer, and trigger defects such as reduced pressure resistance and electrical shorts. In addition, when an Al alloy film in the related art is directly bonded to an oxide conducting film such as ITO, diffusion reactions occur in the interface and deteriorate electrical characteristics. Furthermore, the Al alloy film in the related art generally causes strong mutual diffusion reactions in the bonding interface with a Si semiconductor film or a film containing Si as the main ingredient, and deteriorates electrical characteristics. Therefore, when the Al alloy film is to be connected to ITO and Si, the Al alloy film cannot be directly connected to them. Instead, the Al alloy film needs to be connected to them by using above-mentioned high melting point metal interposed therebetween as a barrier layer. As a result, it has raised a problem that the number of processes including a film-forming process and an etching process has increased and productivity has decreased.
Accordingly, some improving methods to overcome the above-mentioned problem by contriving effective elements to be added to Al have been proposed. For example, Japanese Unexamined Patent Application Publication No. 7-45555 (Yamamoto et al.) discloses a technique to suppress the occurrence of hillocks by adding a rare-earth element such as Nd, Gd, and Dy to Al. Japanese Unexamined Patent Application Publication No. 2006-339666 (Goto et al.) discloses a technique to improve the electrical characteristics in the bonding interface with ITO by further adding an element such as Ni, Ag, Zn, and Cu. Japanese Unexamined Patent Application Publication No. 2004-363556 (Ikeda et al.) describes a technique to improve the bonding interfaces with ITO and Si, i.e., semiconductor film, by adding transition metal such as Fe, Co, and Ni to Al.
Incidentally, as flat panel display TVs have been becoming larger and the resolution of compact displays such as the displays of mobile phones has been becoming higher as described above, the need for the improvement of the performance of TFTs as switching elements, let alone for the reduction in the electrical resistance of electrode films, has been also increasing. Therefore, the processing temperature has been required to be reduced in order to minimize the thermal damage to the semiconductor elements constituting TFTs. For example, Japanese Unexamined Patent Application Publication No. 2004-103695 (Nakai et al.) states that the target temperature for accomplishing that purpose is less than 250° C. Alternatively, in reflective display devices or transflective display devices in which organic resin films are used to form reflective pixel electrodes, or in organic resin substrates which are to come into practical use as an alternative to glass substrates to achieve the reduction in both size and weight and to realize curved displays that are believed to become the mainstream in the future, the processing temperature is required to be reduced in order to satisfy the constraint imposed by the heat resistance of those organic resin material. In view of such circumstances, an Al alloy film for which low electrical resistance can be achieved with the processing temperature below 300° C., preferably in the order of 200-250° C. is desired. (For example, Nakai et al. states that it is specifically equal to or less than 10 μΩ·cm, and more preferably equal to or less than 6 μΩ·cm.)
Furthermore, the reduction in the processing temperature is also preferable in that it suppresses the diffusion reactions in the interfaces between the Al alloy film and the ITO film and the Si film. For example, according to the evaluations conducted by the inventor et al. of the present application in which an Al alloy film having the composition disclosed in Ikeda et al. was directly formed on the source and drain electrodes of TFTs using a Si semiconductor, no mutual diffusion reaction was observed in the interface with Si immediately after the film formation. However, the diffusion reaction had gradually proceeded with the heat treatment (it is maintained for about 30 minutes in the ambient atmosphere or nitrogen gas atmosphere), and it had proceeded to such extent at a temperature above 250° C. that the diffusion reaction could be observed even by an optical microscope or the like. Therefore, the processing temperature is also desired to be lowered to a low temperature below 250° C. for that reason.
However, in the case of the Al alloy films disclosed in Yamamoto et al. and Goto et al., the processing temperature needs to be raised to 300° C. or higher in order to achieve sufficiently low electrical resistance. Furthermore, Nakai et al. also states nothing about specific resistance obtained by low temperature heat treatment below 300° C. Accordingly, there has been a problem that the application of an Al alloy film in the related art to the devices in which a low temperature process need be utilized to obtain high-performance TFTs or to make use of organic resin material is very difficult. Furthermore, an Al alloy film is also required to have high reflectivity in the case where it is used for the reflective electrode film of an electrooptic display device. However, substantially no techniques dealing with the reflection characteristics of Al alloy films having the above-mentioned characteristics has been disclosed.
As described above, an Al alloy film in the related art cannot be directly connected to an ITO film or a Si film, and therefore a barrier layer of high melting point metal needs to be formed therebetween. As a result, there have been a problem that the number of processes including a film-forming process and an etching process has increased and productivity has decreased. Furthermore, when Al alloy films disclosed in Yamamoto et al., Goto et al., and Ikeda et al. are used, it requires heat treatment at or above 300° C. in order to achieve sufficiently low electrical resistance. There is a problem that the reduction in the electrical resistance of electrode films is very difficult when it is applied to devices that require a low temperature process at or below 250° C. in order to alleviate the thermal damage to the semiconductors or to satisfy the constraint imposed by the heat resistance of organic resin films that are used for the reflective electrodes.